Focus

Control of gene expression and early development in Drosophila melanogaster

Research

Sex determination

Research on sex determination focuses on the binary switch gene Sex-lethal (Sxl). Sxl plays a pivotal role in the development of sexual dimorphism in the fruit fly. The special importance of Sxl may be understood by considering three of the central themes in development: 1) Pathway initiation: What mechanisms enable a cell to choose between alternative developmental programs? 2) Determination: Once that choice has been made, what mechanisms ensure that that cells remain committed to the correct pathway? 3) Differentiation: What mechanisms ensure the proper elaboration of the developmental program? Sxl is unusual in that it functions in all three of these processes. Additionally Sxl is of interest because its\' developmental functions are controlled by both transcriptional and post-transcriptional regulatory mechanisms. Pathway initiation depends upon a counting system that assess the X chromosome to autosome ratio and regulates the activity of the Sxl-establishment promoter, Sxl-Pe. The Sxl-Pe promoter is turned on female (2X/2A) cells while its kept off in male (1X/2A) cells. Protein products from the Sxl-Pe transcripts in females induce the permanent activation of the Sxl gene by directing the female-specific splicing of transcripts from the Sxl-maintenance promoter, Sxl-Pm. Once activated, the Sxl gene ensures that the female determined state is remembered during the rest development. The memory mechanism is an autoregulatory feed back loop in which Sxl proteins promote their own synthesis by directing the female splicing of Sxl-Pm pre-mRNAs. Finally Sxl orchestrates female differentiation by regulating several gene cascades at the level of splicing and translation.

Chromatin structure and developmental control mechanism

Research on chromatin structure focuses on the regulation of the homeotic Abd-B gene from BX-C. An elaborate cis-regulatory region is responsible for generating parasegment-specific expression of Abd-B. The region controls Abd-B expression in parasegments (PS)10-13 and is subdivided into 4 cis-regulatory domains: iab-5 (PS10), iab-6 (PS11), iab-7 (PS12) and iab-8 (PS13). Abd-B expression is divided into two phases: an initiation phase in which the gap and pair-rule segmentation genes activate the appropriate cis-regulatory domain and a memory phase in which Polycomb group and trithorax group genes maintain the parasegment specific cis-regulatory domains in the correct state, on or off. Each domain contains a unique set of initiator elements that respond to gap/pair-rule genes, and a set of maintenance elements that are targets for the Polycomb and trithorax group proteins. Critical to the functional autonomy of the cis-regulatory domains are chromatin boundary elements that insulate one domain from the adjacent domain. Studies underway are focused on one of the maintenance elements, the iab-7 Polycomb Response Element (PRE), and on two boundary elements Fab-7 and Fab-8 which flank the iab-7 cis-regulatory domain, insulating it from iab-7 and iab-8 respectively. Proteins that are critical for PRE or boundary function have been identified, and we are currently attempting to understand how these proteins contribute to either PRE or boundary activity. In related studies on chromatin structure we have shown that the zw5 gene encodes a protein component of the scs boundary. Multimerized binding sites for the Zw5 protein have boundary activity in vivo, and this activity is dependent on the zw5 gene.

Establishment of polarity in egg and embryo

mRNA localization plays a central role in the establishment of polarity axes in the Drosophila egg and embryo. A key component of the mRNA localization system is encoded by the orb gene. orb is essential for determining both the A-P and D-V axes of the developing egg chamber, and mutations in orb are found to prevent the proper localization of mRNAs required for these polarity axes such as osk, Bic-D, K(10), and gurken. orb encodes a germ-line specific RRM type RNA binding protein which is homologous to the Xenopus CPEB protein. In Xenopus, the CPEB protein controls the translation of masked mRNAs in developing oocytes. We have found that localized mRNAs such as osk, K(10) and Bic-D are not translated in orb mutant ovaries. In favor of a direct role in regulating the localization/translation of these mRNAs, we have found that Orb protein binds to their 3\'UTRs both in vivo and in vitro. One of the mRNAs bound by Orb protein in vivo is orb itself. Recent results indicate that Orb protein autoregulates its own on site expression by binding to localized orb mRNA and activating translation. The orb gene is weakly haploinsufficient and we taken advantage of this haploinsufficiency to devise genetic screens for other genes involved either in orb autoregulation or in the establishment of polarity.